Positive electrode active material for lithium secondary battery comprising lithium metal oxides having multilayered structure and positive electrode comprising the same

ABSTRACT

The present invention provides a positive electrode active material for a lithium secondary battery having a core-shell structure which comprises: a core composed of lithium transition metal oxides including nickel(Ni), manganese(Mn) and cobalt(Co); and a shell composed of lithium transition metal oxides including cobalt(Co), wherein an inorganic material layer is further formed by coating on the surface of the shell.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/KR2016/010445, filed Sep. 20, 2016, published in Korean, which claims priority from Korean Patent Application No. 10-2015-0145825, filed on Oct. 20, 2015, all of which are incorporated herein by reference.

TECHNICAL FIELD

This application claims priority to and the benefit of the benefit of Korean Patent Application No. 10-2015-0145825 filed on Oct. 20, 2015 with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

The present invention relates to a positive electrode active material for a lithium secondary battery comprising lithium metal oxides having a multilayered structure, and a positive electrode comprising the same.

BACKGROUND OF ART

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy and clean energy is increasing, and in a bid to meet the demand, the fields of electric power generation and electric energy storage using electric chemistry are most actively studied.

As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and application thereof is gradually expanding.

Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.

As the lithium secondary battery, lithium-containing cobalt oxide (LiCoO₂) is mainly used, and in addition, the use of lithium-containing manganese oxide such as LiMnO₂ having a layered crystal structure and LiMn₂O₄ having a spinel crystal structure and lithium-containing nickel oxide (LiNiO₂) is also considered.

As the positive electrode active material of a lithium secondary battery, lithium-containing cobalt oxide (LiCoO₂) is mainly used, and in addition, the use of lithium-containing manganese oxide such as LiMnO₂ having a layered crystal structure and LiMn₂O₄ having a spinel crystal structure and lithium-containing nickel oxide (LiNiO₂) is also considered.

Among the above-mentioned positive electrode active materials, LiCoO₂ has excellent lifespan characteristics and charge/discharge efficiency and thus is most frequently used, but it has disadvantages that high temperature safety is lowered, and cobalt used as a raw material is expensive due to its resource restriction and thus there is a limit in price competition aspect.

Lithium manganese oxides such as LiMnO₂ and LiMn₂O₄ are advantageous in that they are excellent in thermal stability, and they are inexpensive and easy to synthesize, but there are problems that the capacity is small, the high temperature characteristics are poor, and the conductivity is low.

In addition, LiNiO₂-based positive electrode active material is relatively inexpensive and shows battery characteristics such as high discharging capacity, but exhibits sudden phase transition of the crystal structure in accordance with volume change accompanying charge/discharge cycles. Further, there is a problem that the stability is abruptly lowered when exposed to air and moisture.

Therefore, recently, a lithium transition metal oxide in which a part of nickel is substituted with another transition metal such as manganese or cobalt has been proposed as an alternative material. However, such metal-substituted nickel-based lithium-transition metal oxides have an advantage in that they have relatively excellent cycle characteristics and capacity characteristics. However, even in this case, the cycle characteristics are drastically lowered when used for a long period of time, and problems such as swelling due to gas generation of the battery and deterioration of thermal stability due to low chemical stability are not sufficiently solved.

Therefore, there is a high need for a positive electrode active material capable of solving the thermal stability problem even while exhibiting improved capacity and output characteristics.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The present invention in some embodiments seeks to resolve the above-described deficiencies of the prior art and the technical issues long outstanding in the art.

As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the present inventors have found that, as described later, when a positive electrode active material for a lithium secondary battery having a core-shell structure includes a core composed of lithium transition metal oxides including nickel(Ni), manganese(Mn) and cobalt(Co), and a shell composed of transition lithium metal oxides including cobalt(Co), wherein an inorganic material layer is further formed by coating on the surface of the shell, the inorganic material layer can enhance the structural stability of the active material, such positive electrode active material can exert only merits of the compounds of each of the core and the shell and thus can maintain the thermal stability while exhibiting high capacity and excellent high output characteristics, thereby completing the present invention.

Technical Solution

In order to achieve the above objects, a positive electrode active material for a lithium secondary battery having a core-shell structure according to the present invention comprises:

a core composed of lithium transition metal oxides including nickel(Ni), manganese(Mn) and cobalt(Co); and

a shell composed of lithium transition metal oxides including cobalt(Co),

wherein an inorganic material layer is further formed by coating on the surface of the shell, and

wherein the content of the inorganic material layer is 0.1 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material.

Therefore, the positive electrode active material for a lithium secondary battery according to the present invention is formed in a structure including a core composed of lithium transition metal oxides including nickel(Ni), manganese(Mn) and cobalt(Co), and a shell composed of lithium transition metal oxides including cobalt(Co), wherein an inorganic material is further formed by coating on the surface of the shell, whereby such a positive electrode active material can improve the lifetime of the secondary battery, supplement defects of the compounds of the core and the shell from each other and exert only merits of each compound, thereby maintaining the thermal stability while exhibiting high capacity and excellent high output characteristics.

In one specific embodiment, in order to enhance the structural stability, the positive electrode active material for a lithium secondary battery according to the present invention may have a form in which at least one or more of the transition metals in the lithium transition metal oxides can substituted with one or more metals having a valence of +2 or +3 within a predetermined amount range. Specifically, the lithium transition metal oxide of the core and the lithium transition metal oxide of the shell may further include one or more elements which are in each case mutually independently selected from the group consisting of tungsten(W), boron(B), aluminum(Al), zirconium(Zr), titanium(Ti), magnesium(Mg), chromium(Cr), and silicon(Si). More specifically, the lithium transition metal oxide of the core and the lithium transition metal oxide of the shell may further include zirconium(Zr) element.

Specifically, the lithium transition metal oxide of the core may be a compound represented by the following Formula 1, and the lithium transition metal oxide of the shell may be a compound represented by the following Formula 2. Li_(y)[Ni_(a)Mn_(b)Co_(1−(a+b+c))M_(c)]O₂  (1)

wherein,

0.55≤a≤0.9, 0.05≤b≤0.5, 0≤c≤0.1, a+b+c≤1, 0.98≤y≤1.10; and

M is one or more selected from the group consisting of W, B, Al, Zr, Ti, Mg, Cr, and Si; Li_(y)Co_((1−a))Z_(a)O₂  (2)

wherein,

0≤a≤0.1, 0.98≤y≤1.10; and

Z is one or more selected from the group consisting of W, B, Al, Zr, Ti, Mg, Cr, and Si.

It is desirable that the lithium transition metal oxide of Formula 1 contains a relatively large amount of Ni relative to Mn and Co, thereby maximizing the capacity. Therefore, the molar fraction(a) of Ni may be 0.55 to 0.9. On the other hand, when the content of nickel is less than 0.5, it is difficult to expect a high capacity, and when the content exceeds 0.95, there is a problem that structural stability within the cycle is extremely deteriorated.

It is desirable that the lithium transition metal oxide of Formula 2 contains a relatively large amount of Co, thereby improving high output characteristics. Therefore, the molar fraction (1-a) of Co may be 0.9 to 1.0. In one specific embodiment, the inorganic material layer may include one or more inorganic element selected from the group consisting of Al, Ti, Zr, W, Mg, Co, B and Nb. Specifically, the inorganic material layer may include an inorganic element of Al.

In one specific embodiment, the inorganic material layer may be any one selected from the group consisting of Al₂O₃, ZrO₂, PO₄, SeO₃, and SnO₂.

In addition, the content of the inorganic material layer may be 0.1 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material. On the other hand, when the content of the inorganic material layer is less than 0.1 wt %, it is difficult to improve the stability of the positive electrode active material, and when it exceeds 3.0 wt %, the energy capacity may decrease. Specifically, the content of the inorganic material layer may be 1.1 wt % or more to 3.0 wt % or less, more specifically, 2.0 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material.

In one specific embodiment, the inorganic material layer may have a thickness of 1 to 150 nm.

In one specific embodiment, the content ratio between the core and the shell may be 20:80 to 80:20, specifically 40:60 to 60:40 on a weight basis. More specifically, the content ratio between the core and the shell may be 40:60 to 50:50, or 50:50 to 60:40. When the content ratio is out of the range of 20:80 to 80:20, it is difficult to exhibit high capacity and excellent high output characteristics because it is difficult to supplement defects of the compounds of the core and the shell from each other and exert only merits of each compound, thereby making it to exhibit high capacity and excellent high output characteristics.

In one specific embodiment, the core may have a concentration gradient in which the concentration of nickel, manganese, and cobalt decreases toward the outer surface, and the shell has a concentration gradient in which the concentration of cobalt decreases toward the outer surface.

In another specific embodiment, the core may have a structure in which the concentration of nickel, manganese, and cobalt is constant without a concentration gradient within the core region, and the shell can have a structure in which the concentration of cobalt is constant without a concentration gradient within the shell region.

The positive electrode active material may further include a concentration gradient layer formed between the outside of the core and the inside of the shell, in which the concentration of one or more transition metals continuously changes to exhibit a concentration gradient.

The transition metal can be one or more selected from the group consisting of nickel, manganese and cobalt.

Further, the particle size of the positive electrode active material can be 0.1 to 1 μm.

In one specific embodiment, the positive electrode active material is a secondary particle formed by aggregation of primary particles, and a lithium ion diffusion path in the primary particle may be formed in the center direction of the secondary particle.

The present invention may also provide a method for producing the positive electrode active material for a lithium secondary battery.

Specifically, the method for producing the positive electrode active material for a lithium secondary battery comprises:

(a) uniformly mixing a precursor for the production of a positive electrode active material having a core-shell structure with lithium oxide, and then performing a calcination reaction to produce a lithium transition metal oxide powder.

Here, the lithium oxide may be lithium carbonate (Li₂CO₃) and/or lithium hydroxide (LiOH).

The method for producing the positive electrode active material for a lithium secondary battery according to the present invention may comprise the steps of:

(b) preparing a mixed solution containing one or more inorganic element selected from the group consisting of Al, Ti, Zr, W, Mg, Co, B and Nb in an organic solvent;

(c) mixing the mixed solution with the prepared lithium transition metal oxide powder and stirring the mixed solution in a paste state;

(d) drying the pasty mixture to evaporate the organic solvent; and

(e) heat-treating the dried mixture to produce a positive electrode active material in which an inorganic material layer is formed on the surface of the lithium transition metal oxide particles.

Further, the present invention provides a positive electrode including the positive electrode active material, and provides a lithium secondary battery including the positive electrode.

On the other hand, the lithium secondary battery is generally composed of the positive electrode, a negative electrode, a separator, and a lithium salt-containing non-aqueous electrolyte.

The positive electrode is produced, for example, by coating a mixture slurry of the positive electrode active material according to the present invention, a conductive material and a binder onto a positive electrode current collector, and then drying the result, and if necessary, the mixture (electrode mixture) of the positive electrode active material, a conductive material, a binder and the like may further include at least one material selected from the group consisting of a viscosity controlling agent and a filler.

The positive electrode current collector is generally produced to have a thickness of 3 to 500 μm. The positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical change in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, calcined carbon; or aluminum; or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver or the like. The current collector can form fine unevenness on its surface to increase the adhesive force of the positive electrode active material and can take various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric and the like.

The conductive material is a component for further improving the conductivity of the electrode active material, and can be added in an amount of 0.01 to 30% by weight based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has a conductivity without inducing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; carbon fiber such as carbon nanotube and fullerene; conductive fiber such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskey such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives; and the like.

The binder is a component that assists in bonding between an active material and a conductive material and in binding with a current collector. The binder may be typically added in an amount of 1 to 50% by weight based on the total weight of a mixture containing a positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, a styrene-butadiene rubber, a fluorine rubber, various copolymers, and the like.

The viscosity controlling agent is a component for controlling the viscosity of an electrode mixture so as to facilitate a mixing process of an electrode mixture and a process of coating the same onto a current collector, and may be added in an amount of up to 30 wt % based on the total weight of the electrode mixture. Examples of such a viscosity controlling agent include carboxymethylcellulose, polyvinylidene fluoride, and the like, but is not limited thereto. In some cases, the solvents described above may also function as the viscosity controlling agent.

The filler is optionally used as an auxiliary component for suppressing the expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without inducing chemical change in the battery. Examples of the filler include olefin-based polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode may be produced by coating a negative electrode material onto a negative electrode current collector and drying the result, and if necessary, components such as a conductive material, a binder and the like as described above may also be further included. The negative electrode current collector is generally produced to have a thickness of 3 to 500 μm. The positive electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, similarly to the positive electrode current collector, fine unevenness may be formed on the surface to enhance the adhesive force of the negative electrode active material, and it may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The negative electrode active material may include, for example, carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitized carbon, carbon black, carbon nanotube, fullerene and activated carbon; metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti or the like that can be alloyed with lithium, and compounds containing these elements; complexes of metals and their compounds with carbon and graphite materials; lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used alone or in combination of two or more.

The separator is interposed between the positive electrode and the negative electrode, and an insulating thin film having high ion permeability and mechanical strength is used as the separator. The separator typically has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator, sheets or non-woven fabrics made of an olefin-based polymer such as polypropylene, glass fibers or polyethylene, which have chemical resistance and hydrophobicity, are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as both the separator and electrolyte.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. The non-aqueous electrolyte that can be used includes a non-aqueous electrolyte solution, an organic solid electrolyte, an inorganic solid electrolyte, and the like. Examples of the non-aqueous organic solvent may include an aprotic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfranc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate and the like. Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, polymers containing ionic dissociation groups, and the like.

Examples of the inorganic solid electrolyte include nitrides, halides, sulfates or the like of Li such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH, Li₃PO₄—Li₂S—SiS₂, or the like.

The lithium salt is a substance that is readily soluble in the non-aqueous electrolyte, and example thereof include LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, imide and the like.

In addition, in order to improve charge/discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, or the like may be added to the non-aqueous electrolyte. In some cases, in order to impart incombustibility, the electrolyte may further include a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride. In addition, in order to improve high-temperature storage characteristics, the electrolyte may further include carbon dioxide gas, and it may further include FEC (fluoro-ethylene carbonate), PRS (propene sultone), FPC (fluoro-propylene carbonate), or the like.

The present invention also provides a battery pack including a lithium secondary battery.

Since the structure and manufacturing method of such a battery pack are well known in the art, a detailed description thereof will be omitted herein.

The battery pack may be included as a power source of a mobile device which requires high capacity, excellent output characteristics, and battery safety.

Specifically, the device may be selected from the group consisting of, for example, a mobile phone, a portable computer, a wearable electronic device, a tablet PC, a smart pad, a netbook, and a smart watch.

Since the structure and manufacturing method of such a device are well known in the art, a detailed description thereof will be omitted herein.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Now, the present invention will be described in more detail with reference to the following examples, comparative examples and experimental example, but the scope of the present invention is not limited thereto.

Example 1

(Production of Precursor for Production of Positive Electrode Active Material)

In order to produce a precursor for the production of a positive electrode active material for a secondary battery, first, 2 L of distilled water was filled in a 3 L wet-type reactor tank, and then nitrogen gas was continuously injected into the tank at a rate of 1 L/min to remove dissolved oxygen. At this time, the temperature of the distilled water in the tank was maintained at 45 to 50° C. by using a temperature maintaining device. Further, the distilled water in the tank was stirred with speeds in the range of 1000-1200 rpm using an impeller connected to a motor installed outside the tank.

Nickel sulfate, manganese sulfate, and cobalt sulfate were mixed at a ratio (molar ratio) of 0.55:0.25:0.2 to prepare a 1.5M transition metal aqueous solution. In addition, a 3M sodium hydroxide aqueous solution was also prepared. The transition metal aqueous solution was added at a rate of 0.3 L/hr to a wet-type reactor maintained at 45 to 50° C. and containing distilled water, and a salt containing zirconium (Zr) as a doping element was added. The prepared sodium hydroxide aqueous solution was added so that the distilled water in the wet-type reactor was maintained at a pH of 11.0 to 12.5. A 30% ammonia solution as an additive was continuously to the wet-type reactor at a rate of 0.035 L to 0.04 L/hr. The mixture was stirred using an impeller speed of 1100 rpm, and then the transition metal aqueous solution, the sodium hydroxide aqueous solution and the ammonia solution were added by adjusting their flow rates such that the average residence time of the solutions in the wet-type reactor was about 5 hours. After the reaction in the reactor arrived at a steady state, a certain duration of time was given to obtain an oxide with a higher density. Then, transition metal hydroxide (Ni_(0.55)Mn_(0.3)Co_(0.1)Zr_(0.05)(OH_(0.53))₂) was synthesized.

To the transition metal hydroxide (Ni_(0.55)Mn_(0.3)Co_(0.1)Zr_(0.05)(OH_(0.53))₂) synthesized in the wet-type reactor, a 2M transition metal aqueous solution containing cobalt sulfate, a salt containing zirconium(Zr) as a doping element, a sodium hydroxide aqueous solution and an ammonia solution were added by adjusting their flow rates such that the average residence time of the solutions in the wet-type reactor was about 2 hours. At this time, the supply of gas was replaced by nitrogen gas to make a reducing atmosphere. A 4M sodium hydroxide solution was added so as to maintain at a pH of 11. After the arrival of a steady state, a certain duration of time was given to obtain an oxide with high density. Subsequently, a shell layer composed of transition metal hydroxide Co_(0.95)Zr_(0.05)(OH_(0.53))₂ was formed on the core composed of Ni_(0.55)Mn_(0.3)Co_(0.1)Zr_(0.05)(OH_(0.53))₂ to obtain a precursor having a core-shell structure.

(Production of Positive Electrode Active Material)

The precursor obtained by the reactor was washed with distilled water, filtered, and dried in a constant temperature drier at 120° C. for 24 hours to remove residual moisture. The thus dried precursor having a core-shell structure and Li₂CO₃ were mixed at a weight ratio of 1:1, and the mixture was heated at a heating rate of 5° C./min and calcined at 920° C. for 10 hours to obtain a lithium transition metal oxide powder (positive electrode active material).

Thereby, the positive electrode active material powder having a core-shell structure in which the core layer of the positive electrode active material was composed of Li[Ni_(0.55)Mn_(0.3)Co_(0.1)Zr_(0.05)]O₂, and the shell layer was composed of LiCo_(0.95)Zr_(0.05)O₂ was obtained. Herein, the content ratio of the core and the shell is 50:50 on a weight basis.

A mixed solution in which a salt containing an Al element was mixed in an organic solvent was mixed with the produced positive electrode active material powder, and stirred in a paste state. the stirred paste-like mixture was dried to evaporate the organic solvent. The thus dried mixture was heat-treated to produce a positive electrode active material in which an aluminum oxide layer (inorganic material layer) was formed on the surface of the positive electrode active material particles in an amount of 2.0 wt % based on the total weight of the positive electrode active material.

(Production of Lithium Secondary Battery)

The previously prepared positive electrode active material was mixed with a conductive material and a binder (PVdF) at a ratio of 95:2.5:2.5 (weight ratio of active material, conductive material and binder) and the mixture was added to NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a positive electrode mixture slurry. 95 wt % of an artificial graphite as a negative electrode active material, 1.5 wt % of a conductive material (Super-P) and 3.5 wt % of a binder (PVdF) were added to a solvent NMP to prepare a negative electrode mixture slurry. Then, coating, drying and pressing were respectively carried out on the aluminum foil and the copper foil to produce a positive electrode and a negative electrode.

A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and then an electrolytic solution in which 1 M LiPF₆ was dissolved in a carbonate solvent having EC:EMC=1:2 was injected to produce a coin battery.

Example 2

A battery was produced in the same manner as in Example 1, except that a 1.5M transition metal aqueous solution was prepared by mixing nickel sulfate, manganese sulfate, and cobalt sulfate at a ratio (molar ratio) of 0.6:0.2:0.2.

Example 3

A battery was produced in the same manner as in Example 1, except that a salt containing zirconium (Zr) as a doping element was not supplied during the production of a precursor for the production of a positive electrode active material for a secondary battery.

Example 4

A battery was produced in the same manner as in Example 1, except that an inorganic material layer was not formed on the surface of the positive electrode active material having a core-shell structure.

Example 5

A battery was produced in the same manner as in Example 1, except that the content of the inorganic material layer formed on the surface of the positive electrode active material was 0.1 wt % based on the total weight of the positive electrode active material.

Example 6

A battery was produced in the same manner as in Example 1 except that the content of the inorganic material layer formed on the surface of the positive electrode active material was 3.0 wt % based on the total weight of the positive electrode active material.

Example 7

A battery was produced in the same manner as in Example 1 except that the content of the inorganic material layer formed on the surface of the positive electrode active material was 1.1 wt % based on the total weight of the positive electrode active material.

Comparative Example 1

A battery was produced in the same manner as in Example 1, except that a positive electrode active material powder having a structure in which compound particles of Li[Ni_(0.55)Mn_(0.3)Co_(0.1)Zr_(0.05)]O₂ and compound particles of LiCo_(0.95)Zr_(0.05)O₂ were uniformly mixed was prepared, instead of preparing a positive electrode active material powder having a core-shell structure.

Comparative Example 2

A battery was produced in the same manner as in Example 1 except that the content of the inorganic material layer formed on the surface of the positive electrode active material was 5.0 wt % based on the total weight of the positive electrode active material.

Comparative Example 3

A battery was produced in the same manner as in Example 1 except that the content of the inorganic material layer formed on the surface of the positive electrode active material was 0.05 wt % based on the total weight of the positive electrode active material.

Experimental Example 1

Lifespan Characteristics

The coin batteries respectively produced in Examples 1 to 11 and Comparative Examples 1 to 3 were charged and discharged 100 times with a current of 0.5 C in a voltage range of 3.0V to 4.4V to evaluate the lifespan characteristics. The results are shown in Table 1 below.

TABLE 1 lifespan characteristic 30^(th)/1^(st) discharging capacity Sample (%) Example 1 98% Example 2 97% Example 3 96% Example 4 96% Example 5 98% Example 6 98% Example 7 98% Comparative 93% Example 1 Comparative 92% Example 2 Comparative 92% Example 3

As shown in Table 1, it was confirmed that the lithium secondary battery using the positive electrode active material of the core-shell structure produced in Examples 1 to 7 according to the present invention exhibited a high capacity maintenance rate as compared with the lithium secondary batteries of Comparative Examples 1 to 3.

Experimental Example 2

High-Speed Charging Characteristics

The coin batteries respectively produced in Examples 1 to 7 and Comparative Examples 1 to 3 were charged and discharged with a current of 0.1 C in a voltage range of 3.0V to 4.4V and then charged and discharged with a current of 5.0 C to evaluate high-speed charging characteristics. The results are shown in Table 2 below.

TABLE 2 High-speed 0.1 C charging 5 C charging charging capacity capacity efficiency Sample (mAh/g) (mAh/g) 5.0 C/0.1 C (%) Example 1 184 165 90 Example 2 186 168 90 Example 3 185 161 87 Example 4 185 160 86 Example 5 184 165 90 Example 6 184 165 90 Example 7 185 165 90 Comparative 184 155 84 Example 1 Comparative 185 152 82 Example 2 Comparative 181 151 85 Example 3

As shown in Table 2, it was confirmed that the lithium secondary batteries produced in Examples 1 to 7 according to the present invention exhibited high output characteristics as compared with the lithium secondary batteries of Comparative Examples 1 to 3.

Experimental Example 3

Measurement of Thermal Stability Through DSC

In a state where the positive electrodes containing the respective active materials produced in Example 1, Example 5, Example 6 and Example 7, Comparative Example 2 and Comparative Example 3 were charged at 4.4V, and the thermal stabilities were measured while raising the temperature at a rate of 10° C./min using a differential scanning calorimeter (DSC). The results are shown in Table 3 below.

TABLE 3 Sample DSC Temperature(° C.) Calorie (J/g) Example 1 297 985 Example 5 279 1120 Example 6 294 991 Example 7 291 1008 Comparative 282 1107 Example 2 Comparative 275 1136 Example 3 As can be seen from the results in Table 3, it was confirmed that Example 1, Example 6 and Example 7 exhibited superior thermal stability as compared with Example 5, Comparative Example 2 and Comparative Example 3.

As described above, the positive electrode active material for a lithium secondary battery according to the present invention is formed in a structure including a core composed of lithium transition metal oxides including nickel(Ni) and manganese(Mn) and cobalt(Co), and a shell composed of lithium transition metal oxides including cobalt(Co), wherein an inorganic material layer is further formed by coating on the surface of the shell, whereby the inorganic material layer can enhance the structural stability of the active material, supplement defects of the compounds of the core and the shell from each other and exert only merits of each compound, thereby maintaining the thermal stability while exhibiting high capacity and excellent high output characteristics.

Various changes and modifications within the sprit and scope of the invention will become apparent to those skilled in the art from this detailed description.

INDUSTRIAL APPLICABILITY

As described above, the positive electrode active material for a lithium secondary battery according to the present invention is formed in a structure including a core composed of lithium transition metal oxides including nickel(Ni) and manganese(Mn) and cobalt(Co), and a shell composed of lithium transition metal oxides including cobalt(Co), wherein an inorganic material layer is further formed by coating on the surface of the shell, whereby the inorganic material layer can enhance the structural stability of the active material, supplement defects of the compounds of the core and the shell from each other and exert only merits of each compound, thereby maintaining the thermal stability while exhibiting high capacity and excellent high output characteristics. 

What is claimed is:
 1. A positive electrode active material for a lithium secondary battery having a core-shell structure comprising: a core composed of lithium transition metal oxides including nickel(Ni), manganese(Mn) and cobalt(Co); and a shell composed of lithium transition metal oxides including cobalt(Co), wherein the lithium transition metal oxide of the core is a compound represented by the following Formula 1, and the lithium transition metal oxide of the shell is a compound represented by the following Formula 2: Li_(y)[Ni_(a)Mn_(b)Co_(1−(a+b+c))M_(c)]O₂  (1) wherein, 0.55≤a≤0.9, 0.05≤b≤0.5, 0<c≤0.1, a+b+c≤1, 0.98≤y≤1.10; and M is Zr or Zr and one or more selected from the group consisting of W, B, Al, Ti, Mg, Cr, and Si; Li_(y)Co_((1−a))Z_(a)O₂  (2) wherein, 0<a≤0.1, 0.98≤y≤1.10; and Z is Zr or Zr and one or more selected from the group consisting of W, B, Al, Zr, Ti, Mg, Cr, and Si, wherein an inorganic material layer is further formed by coating on a surface of the shell, and wherein a content of the inorganic material layer is 0.1 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material.
 2. The positive electrode active material according to claim 1, wherein the inorganic material layer includes one or more inorganic elements selected from the group consisting of Al, Ti, Zr, W, Mg, Co, B and Nb.
 3. The positive electrode active material according to claim 1, wherein the inorganic material layer is any one selected from the group consisting of Al₂O₃, ZrO₂, PO₄, SeO₃, and SnO₂.
 4. The positive electrode active material according to claim 1, wherein the inorganic material layer includes an inorganic element of Al.
 5. The positive electrode active material according to claim 1, wherein the content of the inorganic material layer is 1.1 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material.
 6. The positive electrode active material according to claim 5, wherein the content of the inorganic material layer is 2.0 wt % or more to 3.0 wt % or less based on the total weight of the positive electrode active material.
 7. The positive electrode active material according to claim 1, wherein the inorganic material layer has a thickness of 1 to 150 nm.
 8. The positive electrode active material according to claim 1, wherein a content ratio between the core and the shell is 20:80 to 80:20 on a weight basis.
 9. The positive electrode active material according to claim 1, wherein the core has a concentration gradient in which a concentration of nickel, manganese, and cobalt decreases toward the outer surface, and the shell has a concentration gradient in which a concentration of cobalt decreases toward the outer surface.
 10. The positive electrode active material according to claim 1, wherein the core has a structure in which a concentration of nickel, manganese, and cobalt is constant without a concentration gradient within a core region, and the shell has a structure in which a concentration of cobalt is constant without a concentration gradient within a shell region.
 11. The positive electrode active material according to claim 1, wherein the positive electrode active material further includes a concentration gradient layer formed between an outside of the core and an inside of the shell, in which a concentration of one or more transition metals continuously changes to exhibit a concentration gradient.
 12. The positive electrode active material according to claim 1, wherein the positive electrode active material has a particle size of 0.1 to 1 μm.
 13. The positive electrode active material according to claim 1, wherein the positive electrode active material is a secondary particle formed by aggregation of primary particles, and a lithium ion diffusion path in the primary particle is formed in the center direction of the secondary particle.
 14. A method for producing the positive electrode active material for a lithium secondary battery of claim 1 comprising: (a) uniformly mixing a precursor for the production of a positive electrode active material having a core-shell structure with lithium oxide, and then performing a calcination reaction to produce a lithium transition metal oxide powder.
 15. The method according to claim 14, wherein the method comprises the steps of: (b) preparing a mixed solution containing one or more inorganic element selected from the group consisting of Al, Ti, Zr, W, Mg, Co, B and Nb in an organic solvent; (c) mixing the mixed solution with the prepared lithium transition metal oxide powder and stirring the mixed solution in a paste state; (d) drying the pasty mixture to evaporate the organic solvent; and (e) heat-treating the dried mixture to produce a positive electrode active material in which a inorganic material layer is formed on the surface of the lithium transition metal oxide particles.
 16. A positive electrode comprising the positive electrode active material according to claim
 1. 17. A lithium secondary battery comprising the positive electrode according to claim
 16. 